This invention relates to computer printers, and particularly to media transports mechanisms and vacuum hold-down devices.
Some approaches for thermal inkjet printing use a vacuum platen as part of the media transport. Essentially, a sheet of media to be printed is carried on an air-transmissive belt over a flat plate that contains a multitude of apertures. A vacuum device below the plate draws air into the apertures, creating a pressure differential that flattens the media sheet against the plate, with the web sliding over the plate to feed the sheet past a printing device. The printing device may be a thermal ink jet pen that reciprocates over the sheet in a scan direction perpendicular to the feed direction, and which lays down successive swaths of ink droplets to generate a printed image.
The platen is normally heated to facilitate rapid drying of aqueous ink, and the vacuum effect holds the sheet in a flat stable position as the ink dries. This avoids curling or xe2x80x9ccocklexe2x80x9d effects that can distort the media surface in areas where large quantities of ink are imprinted, due to the dimensional effect of moisture on paper and other media. When the media is held flat during the drying process, a flat result is generated.
While effective for many applications, vacuum platens have certain limitations. First, smaller media that does not cover most of the platen area leave substantial platen areas open. This permits air to be drawn into the area below the platen, bypassing the sheet, and thereby requiring substantial airflow capacity to maintain adequate relative pressure on the sheet. For a minimally sized sheet, nearly the entire area of the platen may be open to airflow. This requires a large vacuum blower, with attendant problems of size, power consumption, and noise. Further, for the platen to be maintained at an elevated temperature needed for ink drying, increased heating power is needed to offset the cooling effect of ambient air flowing through the platen. Also, open areas surrounding a small media sheet may still have depressed temperatures compared to covered regions, and subsequent large media may encounter non-uniform platen temperatures that may impair printing results.
An additional concern even for platens optimized for a particular media width is that unless a continuous end-to-end stream of media is passed over the platen, there will be large open areas of the platen ahead of the leading edge of the first sheet, and following the training edge of the last sheet. This generates similar disadvantages to those discussed above regarding media width.
The present invention overcomes the limitations of the prior art by providing a printer having a media transport with a rigid, air-transmissive platen. A movable air-transmissive flexible web overlays the platen, and a suction device communicates with the platen to draw air through the web and through the platen, such that a sheet of media carried on the web is biased toward the platen. A second movable web below the first web limits air flow through at least a selected portion of the platen other than a portion overlaid by the sheet. The movable web may be a continuous belt, which may have two separate air-blocking regions that are positioned beyond the platen when full airflow is desired, and at least partly registered with the platen to block airflow to peripheral platen portions.